Results of intestinal transplantation are inferior to other transplanted organs primarily due to the greater immunogenicity of intestinal allografts and complications arising from the large amount of immunosuppression required to prevent rejection. These facts underscore the need to understand the unique mechanisms responsible for intestinal allograft rejection and to use this knowledge to design more effective immunosuppression. Studies of other transplanted organs demonstrate an important role for recipient secondary lymphoid organs in the rejection process. We hypothesize that mesenteric lymph nodes (MLN) and Peyer's patches (PP) within intestinal allografts serve as sites that uniquely and efficiently prime alloreactive T cells. By virtue of having been primed in the lymphoid organs of the intestine, recipient T cells express trafficking molecules that favor their subsequent migration to the intestinal epithelium where they mediate rejection. Specific aim 1 is to determine whether priming of recipient alloreactive T cells can occur in donor MLN and PP and if so, the contribution this process to intestinal rejection relative to the more conventional process of T cell priming in recipient lymphoid organs. For these experiments mutant mice that lack all lymph nodes and PP (LTalpha-/- and aly/aly) will be used as recipients and/or donors for intestinal allografts. T cell priming in donor and recipient lymphoid organs will be assessed by Elispot and intracellular cytokine staining. Specific aim 2 is to determine whether chemokines (CCR6 and CCR9) and integrins (CD103 and alpha4beta7) known to regulate intestinal T cell trafficking in response to environmental and microbial antigens also regulate the trafficking of recipient T cells in response to intestinal alloantigens. The role of these molecules in T cell trafficking to intestinal allografts will be examined using knockout mice and monoclonal antibodies specific for each molecule. The effects of each molecule will be quantified using intravital microscopy, accumulation of In(111) labeled cells, and flow cytometric analysis of T cells isolated from the various compartments of intestinal grafts. Specific aim 3 is to combine agents that block those steps shown to be important in aims 1 and 2 with other small molecule or biologic immunosuppressant agents in a attempt to design regimens that more specifically and effectively prevent the rejection of intestinal allografts.